Evaporation of gasoline fuel from motor vehicle fuel systems is a major potential source of hydrocarbon air pollution. Such emissions can be controlled by the canister systems that employ activated carbon to adsorb the fuel vapor emitted from the fuel systems. Under certain modes of engine operation, the adsorbed fuel vapor is periodically removed from the activated carbon by purging the canister systems with ambient air to desorb the fuel vapor from the activated carbon. The regenerated carbon is then ready to adsorb additional fuel vapor.
An increase in environmental concerns has continued to drive strict regulations of the hydrocarbon emissions from motor vehicles even when the vehicles are not operating. When a vehicle is parked in a warm environment during the daytime heating (i.e., diurnal heating), the temperature in the fuel tank increases resulting in an increased vapor pressure in the fuel tank. Normally, to prevent the leaking of the fuel vapor from the vehicle into the atmosphere, the fuel tank is vented through a conduit to a canister containing suitable fuel adsorbent materials that can temporarily adsorb the fuel vapor. The fuel vapor from the fuel tank enters the canister through a fuel vapor inlet of the canister and diffuses into the adsorbent volume where it is adsorbed in temporary storage before being released to the atmosphere through a vent port of the canister. Once the engine is turned on, ambient air is drawn into the canister system through the vent port of the canister. The purge air flows through the adsorbent volume inside the canister and desorbs the fuel vapor adsorbed on the adsorbent volume before entering the internal combustion engine through a fuel vapor purge conduit. The purge air does not desorb the entire fuel vapor adsorbed on the adsorbent volume, resulting in a residue hydrocarbon (“heel”) that may be emitted to the atmosphere. In addition, that heel in local equilibrium with the gas phase also permits fuel vapors from the fuel tank to migrate through the canister system as emissions. Such emissions typically occur when a vehicle has been parked and subjected to diurnal temperature changes over a period of several days, commonly called “diurnal breathing losses.” The California Low Emission Vehicle Regulation makes it desirable for these diurnal breathing loss (DBL) emissions from the canister system to be below 10 mg (“PZEV”) for a number of vehicles beginning with the 2003 model year and below 50 mg, typically below 20 mg, (“LEV-II”) for a larger number of vehicles beginning with the 2004 model year. Now the California Low Emission Vehicle Regulation (LEV-III) requires canister DBL emissions not to exceed 20 mg as per the Bleed Emissions Test Procedure (BETP) as written in the California Evaporative Emissions Standards and Test Procedures for 2001 and Subsequent Model Motor Vehicles, Mar. 22, 2012.
Several approaches have been reported to reduce the diurnal breathing loss (DBL) emissions. One approach is to significantly increase the volume of purge gas to enhance desorption of the residue hydrocarbon heel from the adsorbent volume. This approach, however, has the drawback of complicating management of the fuel/air mixture to the engine during purge step and tends to adversely affect tailpipe emissions. See U.S. Pat. No. 4,894,072.
Another approach is to design the canister to have a relatively low cross-sectional area on the vent-side of the canister, either by the redesign of existing canister dimensions or by the installation of a supplemental vent-side canister of appropriate dimensions. This approach reduces the residual hydrocarbon heel by increasing the intensity of purge air. One drawback of such approach is that the relatively low cross-sectional area imparts an excessive flow restriction to the canister. See U.S. Pat. No. 5,957,114.
Another approach for increasing the purge efficiency is to heat the purge air, or a portion of the adsorbent volume having adsorbed fuel vapor, or both. However, this approach increases the complexity of control system management and poses some safety concerns. See U.S. Pat. Nos. 6,098,601 and 6,279,548.
Another approach is to route the fuel vapor through an initial adsorbent volume and then at least one subsequent adsorbent volume prior to venting to the atmosphere, wherein the initial adsorbent volume has a higher adsorption capacity than the subsequent adsorbent volume. See U.S. Pat. No. RE38,844.
The regulations on diurnal breathing loss (DBL) emissions continue to drive new developments for improved evaporative emission control systems, especially when the level of purge air is low. Furthermore, the diurnal breathing loss (DBL) emissions may be more severe for a hybrid vehicle that includes both an internal combustion engine and an electric motor. In such hybrid vehicles, the internal combustion engine is turned off nearly half of the time during vehicle operation. Since the adsorbed fuel vapor on the adsorbents is purged only when the internal combustion engine is on, the adsorbents in the canister of a hybrid vehicle is purged with fresh air less than half of the time compared to conventional vehicles. A hybrid vehicle generates nearly the same amount of evaporative fuel vapor as the conventional vehicles. The lower purge frequency of the hybrid vehicle can be insufficient to clean the residue hydrocarbon heel from the adsorbents in the canister, resulting in high diurnal breathing loss (DBL) emissions.
Accordingly, it is desirable to have an evaporative emission control system with low diurnal breathing loss (DBL) emissions even when a low level of purge air is used, or when the adsorbents in the canister are purged less frequently such as in the case of hybrid vehicles, or both. Though a passive approach has been greatly desired, existing passive approaches still leave DBL emissions at levels that are many times greater than the 20 mg LEV-III requirement when only a fraction of the historically available purge is now available.